Time-resolved Raman spectroscopy of aerosols in a desert environment

Aerosol particles can affect health, climate, and visibility. In desert environments, mechanically lofted crustal dust particles can influence visibility, and can serve as ice nuclei, influencing climate, and can contain pathogens such as valley fever. Understanding the temporal variability in the composition of desert dust particles is challenging in part because of the difficulty of making in-situ measurements of individual particle compositions at time resolutions of one to several hours.

Raman spectroscopy can be used to measure vibrational modes of molecules including those of black carbon, sulfates, nitrates, and crustal materials. The Aerosol Raman Spectrometer (ARS) allows semi-continuous measurement of Raman spectra (RS) of micron-sized aerosol particles with time resolutions of 15 minutes. It measures Raman spectra from particles deposited onto a metallized Mylar tape, sampling particles and measuring RS concurrently. We ran the ARS on the Jornada Experimental Range in New Mexico, USA. The Raman spectra indicated quartz, calcium sulfate, oxalates, and carbonates, as well as other materials such as humic-like substances or soot. The highest numbers and diversity of particles not exhibiting the D/G bands of black carbon occurred shortly before the onset of a thunderstorm-generated dust storm. Many spectra occurred in mixtures with other types of particles. Here we discuss differing fluorescence measurements for similar particle size and intensity and type, and examine the intra-particle variability that was observed at this location. Co-located wind measurements, as well as time-lapse images, are used to understand the processes that may have led to variability in particulate matter composition.